OBJECTIVES: To measure the cavity lumens formed by nucleosomal packing, define the main points of nucleosomal contact, and to define the nucleosomal surfaces in contact with the lumens of the internucleosomal cavities. MAIN METHODS: (a) Passive molecular probes, composed of tritiated glycogen, will be used to explore the cavities within various forms of chromatin. (b) Active probes composed of charged species of hydrolases attached to glycogen with defined molecular diameter will be used to explore the walls of cavities of known diameters.